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I have no background in physics beyond my highschool class, but how I understand gravity is this: matter has inertia, the universe is expanding "up" (into the f

I have no background in physics beyond my highschool class, but how I understand gravity is this: matter has inertia, the universe is expanding "up" (into the future, away from the Big Bang) and out (stretching the surface), the inertia of the matter causes a depression in the spacetime, and the normal force of that curved spacetime causes objects to "attract" each other. This also seems to explain the relationship between gravity and the rate at which time passes (time dilation). Is this understanding wrong by generally accepted science?

deleted 3y ago
[deleted]
pdonis 3y ago
> Is this understanding wrong by generally accepted science? Yes. It's also not relevant to the discussion in this thread. The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.
- Zambyte 3y ago
  > Yes. I was hoping for some elaboration. > The properties of gravity under discussion here have nothing whatever to do with expansion of the universe. At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. Why would matter curve spacetime, if not for the accelerated expansion of spacetime + matter resisting acceleration due to inertia? I'm also OOTL on the boson part (remember, high school physics). I'm asking to hope to understand the subject better.
  - pdonis 3y ago
    > I was hoping for some elaboration. If you mean elaboration on how the expansion of the universe works, I can give that, but as I've said, it's irrelevant to this discussion. > At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. More precisely, the curvature of spacetime around an isolated gravitating body. Which is a very different spacetime geometry from the spacetime geometry of the universe as a whole, which is what cosmologists use to account for the observations that lead us to say that the universe is expanding. > Why would matter curve spacetime, if not for the accelerated expansion of spacetime You have it backwards. What you are calling "the accelerated expansion of spacetime" is an effect of spacetime curvature, not a cause of it. Plus, as I said above, that spacetime curvature is the curvature of the universe as a whole, which is very different from the curvature around an isolated gravitating body, that we are discussing here. > matter resisting acceleration due to inertia? This has nothing to do with spacetime curvature at all. It would be present even if spacetime were flat.
  - pdonis 3y ago
    > I'm also OOTL on the boson part The idea behind that is that, for all other interactions (electromagnetic, weak, strong), our Standard Model of particle physics tells us that there are "gauge bosons" (for electromagnetism it's the photon, for weak it's the W+, W-, and Z bosons, and for strong it's the eight gluons) that mediate the interaction. The full details of how this works involve quantum field theory and are way too long to go into here; all we need for this discussion is the basic idea that any interaction in quantum field theory has one or more "gauge bosons". So if we are going to try to quantize gravity, the obvious way to do it--at least if you are a Standard Model particle physicist--is to treat it the same way and have a "gauge boson" for it. This has actually been done: the quantum field theory of a massless spin-2 field, which is what a "gauge boson" for gravity would look like, was developed in the 1960s and early 1970s. Furthermore, the classical limit of this quantum field theory is known to be General Relativity, i.e., the classical theory of gravity that we already have and that we already know works. So if we want to treat gravity as if it were a Standard Model interaction and give it a "gauge boson", this would be one obvious way to do it. The reddit questioner was asking the more basic question of why we would want to do that at all--why do we need to quantize gravity. If we don't need to, the fact that we could do it using this known mathematical model (there are other issues that come into play with that, which one of the reddit responses mentions, but leave that aside) is irrelevant. Not all mathematical models have physical realizations.
    - Zambyte 3y ago
      Thank you for going deeper. I'm going to give this a more thorough look and investigate deeper when I have more time in a few hours.
    - AnimalMuppet 3y ago
      Probably a dumb question, but... why massless? If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy, and therefore contributes to the mass-energy tensor, further bending spacetime. Would that correspond to a gauge particle with mass?
      - pdonis 3y ago
  - slingnow 3y ago
    You lead with saying you have no background in physics, proceed to give an incorrect explanation, and then asked if it was wrong. Someone simply told you it was in fact wrong -- which apparently didn't meet your expectations. If you wanted someone to explain it to you, why not ask for an explanation or for critique rather than to _expect_ a total stranger to correct all of your understanding out of the goodness of their heart? Why not just come out and state "Someone please offer me their valuable time to explain physics exactly at my level and at my pace. Thanks!"
  - int_19h 3y ago
    > why would matter curve spacetime Perhaps because that's just how matter and spacetime work? I mean, you could ask the same question just about anything in physics. Why is there such a thing inertia at all, for example? You can try explaining things in terms of other things, but at some point you inevitably have to arrive at "it's just how the universe works".
riperoni 3y ago
Well, not really. The expansion of the universe is not responsible for explaining gravity, it was observed and aided in determining models and constants for the theory. Start by looking at a particles property called mass. Mass interacts with the higgs field by exchanging a particle of information called the higgs boson. The interaction strength, corresponding to mass, determines how much the space time around our particle curves. Space time curvature dictates a particles movement. Now throw another particle with mass next to the initial one, also interacting with the higgs field. This is when you get into a huge loop of interacting movements and fields. Locally and classically we approximate it by the classical law of gravity which loosely states that two masses attract each other, depending on how close they are, i.e. GMm/r^2 The universe expanding or contracting is a consequence of mass and gravotational fields, not the other way around. And calculating the expansion and thus curvature of the universe gives you a picture of the average mass distribution.

deleted 3y ago

[deleted]

pdonis 3y ago

> Is this understanding wrong by generally accepted science? Yes. It's also not relevant to the discussion in this thread. The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.

Zambyte 3y ago
> Yes. I was hoping for some elaboration. > The properties of gravity under discussion here have nothing whatever to do with expansion of the universe. At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. Why would matter curve spacetime, if not for the accelerated expansion of spacetime + matter resisting acceleration due to inertia? I'm also OOTL on the boson part (remember, high school physics). I'm asking to hope to understand the subject better.
- pdonis 3y ago
  > I was hoping for some elaboration. If you mean elaboration on how the expansion of the universe works, I can give that, but as I've said, it's irrelevant to this discussion. > At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. More precisely, the curvature of spacetime around an isolated gravitating body. Which is a very different spacetime geometry from the spacetime geometry of the universe as a whole, which is what cosmologists use to account for the observations that lead us to say that the universe is expanding. > Why would matter curve spacetime, if not for the accelerated expansion of spacetime You have it backwards. What you are calling "the accelerated expansion of spacetime" is an effect of spacetime curvature, not a cause of it. Plus, as I said above, that spacetime curvature is the curvature of the universe as a whole, which is very different from the curvature around an isolated gravitating body, that we are discussing here. > matter resisting acceleration due to inertia? This has nothing to do with spacetime curvature at all. It would be present even if spacetime were flat.
- pdonis 3y ago
  > I'm also OOTL on the boson part The idea behind that is that, for all other interactions (electromagnetic, weak, strong), our Standard Model of particle physics tells us that there are "gauge bosons" (for electromagnetism it's the photon, for weak it's the W+, W-, and Z bosons, and for strong it's the eight gluons) that mediate the interaction. The full details of how this works involve quantum field theory and are way too long to go into here; all we need for this discussion is the basic idea that any interaction in quantum field theory has one or more "gauge bosons". So if we are going to try to quantize gravity, the obvious way to do it--at least if you are a Standard Model particle physicist--is to treat it the same way and have a "gauge boson" for it. This has actually been done: the quantum field theory of a massless spin-2 field, which is what a "gauge boson" for gravity would look like, was developed in the 1960s and early 1970s. Furthermore, the classical limit of this quantum field theory is known to be General Relativity, i.e., the classical theory of gravity that we already have and that we already know works. So if we want to treat gravity as if it were a Standard Model interaction and give it a "gauge boson", this would be one obvious way to do it. The reddit questioner was asking the more basic question of why we would want to do that at all--why do we need to quantize gravity. If we don't need to, the fact that we could do it using this known mathematical model (there are other issues that come into play with that, which one of the reddit responses mentions, but leave that aside) is irrelevant. Not all mathematical models have physical realizations.
  - Zambyte 3y ago
    Thank you for going deeper. I'm going to give this a more thorough look and investigate deeper when I have more time in a few hours.
  - AnimalMuppet 3y ago
    Probably a dumb question, but... why massless? If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy, and therefore contributes to the mass-energy tensor, further bending spacetime. Would that correspond to a gauge particle with mass?
    - pdonis 3y ago
- slingnow 3y ago
  You lead with saying you have no background in physics, proceed to give an incorrect explanation, and then asked if it was wrong. Someone simply told you it was in fact wrong -- which apparently didn't meet your expectations. If you wanted someone to explain it to you, why not ask for an explanation or for critique rather than to _expect_ a total stranger to correct all of your understanding out of the goodness of their heart? Why not just come out and state "Someone please offer me their valuable time to explain physics exactly at my level and at my pace. Thanks!"
- int_19h 3y ago
  > why would matter curve spacetime Perhaps because that's just how matter and spacetime work? I mean, you could ask the same question just about anything in physics. Why is there such a thing inertia at all, for example? You can try explaining things in terms of other things, but at some point you inevitably have to arrive at "it's just how the universe works".

riperoni 3y ago

Well, not really. The expansion of the universe is not responsible for explaining gravity, it was observed and aided in determining models and constants for the theory. Start by looking at a particles property called mass. Mass interacts with the higgs field by exchanging a particle of information called the higgs boson. The interaction strength, corresponding to mass, determines how much the space time around our particle curves. Space time curvature dictates a particles movement. Now throw another particle with mass next to the initial one, also interacting with the higgs field. This is when you get into a huge loop of interacting movements and fields. Locally and classically we approximate it by the classical law of gravity which loosely states that two masses attract each other, depending on how close they are, i.e. GMm/r^2 The universe expanding or contracting is a consequence of mass and gravotational fields, not the other way around. And calculating the expansion and thus curvature of the universe gives you a picture of the average mass distribution.